Security systems

ABSTRACT

A security system controls admission of persons to protected areas. An authorised person carries in his pocket a token which generates at two discrete frequencies and when he approaches a door leading to the restricted area, a sensor of his presence sends an initiating signal to a control unit which examines whether the required discrete frequencies are being generated and are being picked up by inductive loops adjacent the door. If the correct signals are being generated the door is automatically opened but if not the door remains closed and an alarm may be given. The user does not need to take his token from his pocket so that the effect will be that the door will open automatically for authorised persons.

United States Patent Lewis et al.

[ June 24, 1975 [54] SECURITY SYSTEMS 3.582.331 EH97; lltlawrtglcki 342/2758 D .639, 06 I97 't .1 I34 75 Inventors: John Lewis, London; William 3,713.83 ,I l' zi 2, Dudley Gilmour, Glastonbury, both 3,732,465 5/1973 Palmer 317/134 of England [73] Assignee: Lewis Security Systems, Ltd., Primary Examiner-Glen R. Swann. ill

London, England Attorney, Agent, or FirmBrumbaugh, Graves. 221 Filed: Nov. 7, 1972 Domhue Raymond 21 A l. N 304,560 I 1 pp 0 57 ABSTRACT [30] Foreign Application Priority Data A security system controls admission of persons to N 8 I971 U d d SW24! protected areas. An authorised person carries in his 6 972 i i 22898/72 pocket a token which generates at two discrete freay mte mg quencies and when he approaches a door leading to the restricted area, a sensor of his presence sends an [52] 340/258 initiating signal to a control unit which examines [51] Int Cl Gosh 13/18 whether the required discrete frequencies are being generated and are being picked up by inductive loops [58] Field of Search fig g 2 1 adjacent the door. if the correct signals are being generated the door is automatically opened but if not the door remains closed and an alarm may be given. The [56] Rehrences cued user does not need to take his token from his pocket UNITED STATES PATENTS so that the effect will be that the door will open auto- 2.774.060 12/ I956 Thompson 340/258 D matically for authorised persons. 3,l68.737 2/[965 Weinstein 343/225 3,344,629 10/1967 Burney 3l7/l34 22 Claims, 3 Drawing Figures 17 13 i A R (I: 31 ---7 I I I U U U SHEET PATENTEDJIJN 24 I975 {lip-GATE DOWN GATE LOW PASS FILTER MIXER 1 10 iii AMPLIFIER} LOCAL A- OSCILLATOR CLOCK GENERA NON -ACCEPT GATE ACCE PT GATE FINAL COUNTER 54 LOGIC UNIT SECURITY SYSTEMS BACKGROUND OF THE INVENTION This invention relates to security systems for controlling the admission of authorised and unauthorised persons to particular areas and to equipment for use with such systems.

SUMMARY OF THE INVENTION According to one aspect of the present invention a security system includes at least one portable token capable of transmitting oscillations at a predetermined frequency, a control unit arranged to detect whether oscillations at the predetermined frequency are being picked up at a certain location and means for operating a deterrent by the control unit. The system is preferably a radio system.

If, for example, there are a number of security areas and some persons are permitted to go in all of them whereas others are only permitted to go into some of them, the persons can have tokens with different transmitters dependent on their degree of authority and a pick-up and radio receiver associated with a particular area and forming part of the control unit can be arranged to control a door or opening constituting the deterrent to restrict entry to persons having transmitters transmitting one frequency, or any one of a number of particular frequencies or those frequencies so modulated by other frequencies. The transmitters can be arranged to be operated continuously so that provided a person has the transmitter on his person he can have access to the area without having to take any action on entry. The transmitters could operate from batteries which are recharged or replaced from time to time and an extension of the system can ensure that no person can take one of the transmitters out of the establishment area, or bring one in.

The detector can be arranged to give an alarm, whether audible or inaudible, that an unauthorised person is approaching the security area or can be arranged to unlock or not unlock a gate or open or not open a door controlling access to the area, or to take a photograph of any unauthorised person, or any combination of these or similar actions.

The system may include a sensor responsive to the presence of a person to initiate a detection cycle in the control unit which will set off an alarm or take other appropriate action if not cancelled by receipt of one of the particular frequencies within the period of the cycle which might be conveniently of the order of one second.

The sensor could be a switch operated by the weight of someone approaching or a photoelectric device or a passive or active acoustic device, or a capacitive device, or a detector using micro-waves or any other form of detector. In an application where the deterrent is a door, the sensor can be constituted by a pick-up of the transmitted signals from the token.

The pick up might use an inductive loop system for picking up transmissions and this could be let into a wall and/or a ceiling in a passage approaching the security area so that it would not matter in what attitude the person was carrying his transmitter.

A single control unit can operate with a number of deterrents and a number of tokens and a number of gates at a number of levels of security.

The control unit includes means such as a local oscillator at a frequency displayed from the predetermined frequencies for providing a local reference signal and means responsive to the picked up signal response and the local reference signal to produce beat frequency pulses and a detector circuit arranged to distinguish between beat frequencies above and below a certain limit.

Preferably the control unit includes a counter and a low pass filter for cutting off frequencies outside a permitted range from the counter, the permitted range of frequencies including the beat frequency. The beat frequency pulses can be counted over a known time interval to see if the received frequency is close enough to the local frequency.

In order to avoid accepting the image frequency, the local frequency can be modulated by a few Hz for a fraction of the detection cycle and not for the remainder of the cycle and beat frequency cycles can be counted in one sense for the fraction of the period and in the other sense for the remainder of the period. The fraction will be preferably one half of the period.

The net count over the examination period will be dependent upon the frequency at which the local oscillator is modulated for the part period and so the detector of the count can distinguish between the beat frequency derived from the correct transmitter frequency and the same beat frequency derived from the image frequency.

BRIEF DESCRIPTION OF THE DRAWING The invention may be carried into practice in various ways and one embodiment will now be described by way of example with reference to the accompanying drawings in which:

FIG. 1 is a diagram illustrating the general principles of a security system embodying the invention;

FIG. 2 is a diagram showing how different pick up loops in the system can be inter-connected to avoid a null, and;

FIG. 3 is a block diagram of some of the components of the control unit shown generally in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The system is for controlling access to different restricted spaces and in the example being described, two doors 11 and 11 are shown diagrammatically in relation to passage 13. The doors are normally locked closed but are to be opened automatically to permit the entry of any person having an appropriate token, but not to be opened to others.

As a person approaches any door I] a sensor 15 detects his presence and sends a signal to a central control unit [6 which is rendered active to examine whether the persons who operated the sensor 15 is carrying a correct token 20. If so, a signal is supplied at 17 to open the door 11 automatically, but if not, the door remains closed and/or an alarm is given and/or a photograph of the person is taken. A second photoelectric sensor (not shown) responsive to a person going through the open doorway can cause an alarm to be given if an unauthorised person passes through after an authorised person before the door has closed again. The authorised persons token will prevent the alarm being given as he goes through.

The signals are detected by an arrangement of inductive loops indicated generally at 18 in the passage by which the door is approached and signals picked up by the loop 18 are fed at 19 to the control unit 16.

ln this way if a user wearing the appropriate token approaches the door, the door automatically opens for him without his having to take any such action as using a key or presenting a coded disc to a checking system; persons requiring access to the door ll say would have similar tokens generally at the same discrete frequencies but if different persons were to be allowed access to the door 1] or to both doors say, then these would have tokens generating at different frequencies, or two tokens at the same frequency for the door 11, one of which transmitted also at the frequency for the door 11'.

The Sensor 1 The sensor may be a photo-electric device or a weight-mat or a door handle switch or an infra-red device or indeed any device that can detect the presence of a person in a particular position and it is only required to supply an activating signal to the control unit.

The Loop System 18 Many different loop systems are possible depending on the particular application but the system being described uses a three-loop system which avoids a null so that it does not matter in what disposition the user is carrying his token.

As shown in FIG. 2, one inductive loop H is arranged to lie in a horizontal plane, and may, for example, be embedded in the floor or ceiling of the passage 13. A second inductive loop V is arranged to lie in a vertical plane which is parallel to the path of people entering the security area, and may, for example, be embedded in the wall of the passage. The third inductive loop D lies in a plane which is mutually perpendicular to the planes containing the loops H and V, and may, for example, encircle the passage, so that a person entering the security area will walk through the inductive loop D.

The system is arranged, as hereinafter described, to admit, or exclude, or give warning of, a person even when only one of the loops is picking up a signal.

Because the loops have a low self-inductance, the signals produced in them by a transmitter will be approximately in phase (or anti-phase). The signal from the loop H passes to a phase shifting circuit 24, which re tards it by a phase angle of 45, while the signal from the loop V passes to a phase shifting circuit 25, which advances it by a phase angle of approx. 45. These two signals are, therefore, now in quadrature. They are then added together by an adder circuit 26. Since the two input signals to the adder are in quadrature, there will always be an output from the adder, unless neither loop H nor loop V is producing a signal.

It is possible that there will be no signal from either of the loops H and V in certain positions of the transmitter, since the aerial of the transmitter is an elementary dipole. In such a case, however, a signal will be picked up by the inductive loop D.

To avoid the possibility of a signal from loop D cancelling out, or nulling, with the signal from the output ofthe adder 26, the signal from loop D is added to the output of adder 26 by an adder circuit 27, and subtracted from it by a subtractor circuit 28. In this way it is ensured that if any ofthe loops H, V and D is receiving a signal, it is impossible for the outputs of both adder 27 and subtractor 28 to be zero.

Two phase shifting circuits 29 and IN and an adder circuits 32, which function in the same way as the circuits 24, 25 and 26 respectively, ensure that an output is produced under all conditions except when there is no output either from adder 27 or from subtractor 28.

The Token This is a multi-frequency oscillator in a sealed plastic cassette to be tamper and shock proof and to be resistant to changes of temperature and humidity. It is operated by a chargable battery carrying a 20 hours charge. It is about the size of a small packet of cigarettes.

A token is arranged to generate signals at 2 discrete frequencies in the range 50 to KHZ and only opens a door 11 designed for these two discrete frequencies. The frequencies need to be at least 20 Hz apart and this gives 5000 possible frequencies so that there are 25,000,000 possible combinations of the two frequencies. The token employs two crystal-controlled oscillators and a ferrite rod aerial. It transmits continuously at the two frequencies, being re-charged at night when not being used. Alternatively it can be a passive generator which is excited into oscillation when it approaches the door by a signal generated in the loop system.

The Control Unit 16 This is in a central consule and receives signals over lines 12 from the sensors 15 disposed throughout the plant in relation to all the doors 11, and from their associated loop system 18, and also controls the transmission of unlocking or alarm signals over lines 17 to the associated doors.

There is a display as indicated at 31 for each of the doors 11, 11 etc, indicating whether a door has been approached by a person and whether or not it accepted or rejected the person. The unit includes a battery charger and a number of lockable sockets indicated generally at 32, into each of which one of the tokens can be inserted at night for recharging the batteries. There is also a check that all tokens have been returned and have not for example been taken home accidentally by the users. The unit may keep a record of the times at which tokens were withdrawn and the times at which they were replaced to keep a record of the hours during which persons were on the premises.

The control unit can provide on a display board a single visible display of the condition of the various doors and of the presence or otherwise of the various tokens and there may also be one or more remote displays controlled from the unit 16.

An additional feature is an exit loop detector at the exits from the premises which gives a signal if any person tries to leave the premises whilst still carrying his token.

The unit 16 may have a pre-programmed timer so that at different times of the day different persons have access to the premises. For example cleaners may be allowed in at certain hours when it is known that the equipment to be protected is separately locked or guarded.

Detecting System The detecting system used in the control unit 16 for each of the doors 11 will now be described in more detail with reference to FIG. 3.

The signals from a pick-up coil 18 are received at 19 by a radio frequency amplifier 35 whose output is fed to two different detecting circuits. one for each of the two discrete frequencies to which that door 11 is to be responsive. Thus the output from the amplifier 35 is fed in parallel to two mixers 36, and 36' each of which also receives an input from a local oscillator 37 or 37'. Since tht two detector circuits operate in precisely the same way except for the particular frequency to which they are responsive, only one of the detectors will be described in detail.

Assuming that the detector to be described is to detect a frequency of 90,040 Hz then the associated local oscillator 37 will be arranged to oscillate at a frequency displaced from that frequency by a few Hz, for example 90,039 Hz and the output from the mixer 36 if the correct frequency is being received at 19 will be a beat frequency of 1 Hz, which is fed to a unit 38 including a low pass filter cutting off at 20 Hz and a pulse shaper. The pulses from 38 are fed in parallel to an up gate 39 and a down gate 4] controlled from a square wave clock generator 42 with a cyclic period of 400 milliseconds, and also from signals received at 43 from the sensor associated with the particular door 11.

A sensors signal at 43 renders the gates 39 and 41 active to count beat pulses from the unit 38, each for the duration of respective 200 milliseconds halves of the square wave from the clock generator 42. This is achieved by an inverter 60 in the connection from the generator 42 to the down gate 41. The clock generator is free running and does not need to be synchronized with the sensor signal.

The local oscillator 37 is also controlled from the clock generator 42 in such a way that the local frequency from oscillator 37 is reduced by 9 Hz during one up part of the detecting cycle which detecting cycle is established by the two, 200 millisecond halves of the square wave from the clock generator 42.

Thus for the up part of the cycle, lasting 200 milliseconds, a received frequency of 90,040 Hz would produce 2 beat pulses, and for a second, down part it would produce 0 beat pulses. If the gate 39, the up gate, counts up to 4 pulses while it is active, it passes a 1 signal to an accept gate 44, but if the count is 5 or more, it passes an 0 signal. The gate 41, the down gate, passes a l signal to a non-accept gate, 45 if it counts 0 2 pulses while it the down gate is active, and passes an 0 signal if it counts more than 2 pulses. The gates 44 and 45 pass logic signals to a final counter 52.

The final counter 52 is switched on at 50 by a pulse from a unit 58 driven the signals from the by clock 42 and triggered by the sensor 43 so that the counting of counter 52 coincides with the signal from the gates 39 and 41.

Beat pulses at a frequency above per second will be filtered out at 38, and therefore incoming frequencies above 90,045 Hz and below 90,010 will not provide the appropriate beat pulses at 39 and 41, and at the final counter.

Incoming frequencies in the range 90,010 to 90,025 and in particular the image frequency 90020 will not product a net count of0 2 in the down gate 41, and so the receiver can be sensitive to the narrow band 90,025 to 90,045.

It will be appreciated that slight variation of the local oscillator frequency or the token oscillator frequency within their tolerance of 5 Hz does not affect this test for the image frequency. Also, white noise from a wide band noise generator would not pass the filter and would produce a net count of 0 at 52. The count system prevents an accept signal due to a l00% amplitudemodulation of sub-sonic frequency.

The final outputs from the counter 52 for both discrete frequencies are examined in a logic unit 54 which is capable of passing an accept signal at 55 to unlock the door 11 over the line 17. There may have been no accept signal because the final count was greater than 4 possibly due to extraneous pulses, or because the incoming frequency was too far displaced from the local oscillator frequency giving a count of 0'; accordingly it is arranged that a second look is automatically made.

Thus if no accept signal appears at 55, a second look signal is fed from the logic unit 54 to the final counter at 61 and when the next signal appears at 50 after a delay of 0.5 seconds sufficient to complete the detecting cycle the unit is reset at 57 and a further detecting cycle is performed to see if this time the correct number of net beat pulses is counted.

In this way a false reject signal or alarm signal will not be given merely because of some extraneous cause which does not repeat itself during the next sampling period.

In an application in a prison, warders could carry the tokens and could have a choice of two frequencies on one token, either of which would open the door, but one of which would also sound an alarm.

What we claim as our invention and desire to secure by Letters Patent is:

l. A security system including at least one portable token embodying a generator and transmitter of oscillations at a predetermined frequency, a pickup at a certain location and means in the pickup for responding to the oscillations transmitted from the token when the token is carried in any atatitude by a user who comes to the location, a sensor responsive to arrival of a person at the location, and a control unit rendered ope rative by response of the sensor and having means for providing a local reference signal, means responsive to the pick-up response and the local reference signal for forming an output signal, and means receiving the output signal for detecting whether the pickup is responding to oscillations at the predetermined frequency, and to deliver a control signal to dependence upon such detection.

2. A system as claimed in claim 1 in which the pickup comprises two inductive loops and the control unit includes phase shifters for shifting the phase of signals picked up by the different loops by different phase angles, and means for combining the phase-shifted signals.

3. A system as claimed in claim 1 in which the token is capable of simultaneously transmitting oscillations at two different predetermined frequencies and including means in the control unit for detecting whether oscillations at both the predetermined frequencies are being picked up by the pickup.

4. A system as claimed in claim 1 including a plurality of tokens at least one of which is capable of transmitting oscillations at at least one predetermined frequency and at least one other of which is capable of transmitting oscillations at at least one different frequency, and means in the control unit for detecting whether the one predetermined frequency is present at one location and whether the different frequency is present at another location.

5. A security system as claimed in claim 4 in which the control unit is arranged to deliver a control signal whether the one or the other frequency is present to be picked up at one of the locations.

6. A system as claimed in claim 1 in which the or each token is arranged to transmit continuously without requiring any action by the person carrying it.

7. A security system as claimed in claim 1 additionally comprising a socket for each token and a battery charger for automatically charging batteries in the tokens when they are inserted in their sockets.

8. A system as claimed in claim I in which the control unit includes a local oscillator oscillating at a frequency displaced from the predetermined frequency for producing beat frequency pulses and a detector circuit for distinguishing between beat frequencies above and below a certain limit.

9. A system as claimed in claim 8 in which the beat frequency pulses are passed through a low pass filter in the control unit for cutting off beat frequencies outside a permitted range.

10. A system as claimed in claim 9 including in the control unit a counter gate for counting beat frequency pulses over a predetermined time interval and means for detecting whether the count achieved is greater or less than a predetermined count.

II. A system as claimed in claim 10 including means for the control unit for detecting whether no pulses are received over an interval and means connected to the counter gate and providing a wave signal for delimiting the inverval.

12. A system as claimed in claim 10 including a logic circuit arranged to give an accept signal for controlling the deterrent in dependence on the count of the counter.

13. A system as claimed in claim 12 wherein the logic circuit includes means for initiating a further detection cycle automatically in response to no accept signal being given within a predetermined time.

14. A system as claimed in claim 13 including means for giving no alarm if, but only if, an acceept signal is given after at least two detection cycles.

15. A system as claimed in claim 8 including in the control unit means for modulating the local oscillator frequency by a few cycles for a fraction of a detecting cycle and not for the remainder of the cycle, means providing a wave signal for delimiting the detecting cycle and means responsive to the wave signal for delimiting fractions of the cycle.

16. A system as claimed in claim l5 including in the control unit gates for counting beat frequency pulses in one sense for the fraction of the cycle and in the other sense for the remainder of the cycle.

17. A security system as claimed in claim 1 including a deterrent and means responsive to the control signal for operating the deterrent.

18. A system as claimed in claim 17 in which the deterrent comprises a door having a lock arranged to be operated in response to the control signal from the control unit.

[9. A system as claimed in claim 17 in which the deterrent comprises an alarm arranged to be operated in response to the control signal from the control unit.

20. A security system as claimed in claim 1 including an exit loop detector at an exit from premises and in which the control unit responds to a signal from the exit loop detector due to a token being taken through the exit.

21. A security system for operating a deterrent comprising:

a portable token for carrying within the system, the

token having means for generating and transmitting oscillations at two different frequencies;

a loop system at a certain location in the system for providing an output signal including the oscillation frequencies from the token, the loop system having two inductive loops in substantially perpendicular planes for picking up the oscillations from the token when the token is carried in any attitude near the location of the loop system and means combining the signals from the loops for providing the output signal; and

a control unit receiving the output signal from the loop system for transmitting a signal operating the deterrent, the control unit having a detecting system, the detecting system having two detecting circuits each receiving the output signal from the loop system and each having a local oscillator arranged to oscillate at a frequency displaced from that of a different one of the token oscillation frequencies in the loop system output signal, means mixing the loop system output signal with the local oscillator frequency for forming beat frequency pulses, and a gate arranged to count the beat frequency pulses during at least part of a signal from a clock for passing a signal having a 0 or 1 state dependent upon the counting of a predetermined number of beat pulses during the clock signal, the control unit also having a clock for providing the clock signal to each gate and means responsive to the state of the signal from each gate for providing the signal operating the deterrent.

22. A system as in claim 21 wherein the means combining the signals from the loops in the loop system comprise means for oppositely shifting the phase of the signal picked up by each loop and means adding each of the oppositely phase-shifted signals for providing the output signals.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTION PATENI NO. 3, 91,9 0

DATED 3 June 24, 1975 INVENTOMS) John Lewis et al it is certified that error appears in the ab0ve-idpntified patent and that saidLetters Patent are hereby corrected as shown beiow:

Col. 2, line 2, "displayed" should read -displaced;

line 48, "passage" should read --passages;

Col. 3, line 8, "Generally" should read -generating-;

Col. 5, line 51, "the signals from the by" should read -by the signals from the-;

Col. 6 line 38, "atatitude" should read --attitude-; and

line 48, "signal to" should read -signal in--;

C01. 7, line 30, "for" (first occurrenoe) should read C01. 4, line 32, "system" should read systems;

Col. 5, line 6, delete the comma after "36" and insert it after "36' line 8, "tht" should read --the-;

line 46, after "gate" delete the comma.

Signed and Scalcd this twenty-first Day Of October 1975 [SEAL] Arrest."

RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner oj'Parents and Trademarks 

1. A security system including at least one portable token embodying a generator and transmitter of oscillations at a predetermined frequency, a pickup at a certain location and means in the pickup for responding to the oscillations transmitted from the token when the token is carried in any atatitude by A user who comes to the location, a sensor responsive to arrival of a person at the location, and a control unit rendered operative by response of the sensor and having means for providing a local reference signal, means responsive to the pick-up response and the local reference signal for forming an output signal, and means receiving the output signal for detecting whether the pickup is responding to oscillations at the predetermined frequency, and to deliver a control signal to dependence upon such detection.
 2. A system as claimed in claim 1 in which the pickup comprises two inductive loops and the control unit includes phase shifters for shifting the phase of signals picked up by the different loops by different phase angles, and means for combining the phase-shifted signals.
 3. A system as claimed in claim 1 in which the token is capable of simultaneously transmitting oscillations at two different predetermined frequencies and including means in the control unit for detecting whether oscillations at both the predetermined frequencies are being picked up by the pickup.
 4. A system as claimed in claim 1 including a plurality of tokens at least one of which is capable of transmitting oscillations at at least one predetermined frequency and at least one other of which is capable of transmitting oscillations at at least one different frequency, and means in the control unit for detecting whether the one predetermined frequency is present at one location and whether the different frequency is present at another location.
 5. A security system as claimed in claim 4 in which the control unit is arranged to deliver a control signal whether the one or the other frequency is present to be picked up at one of the locations.
 6. A system as claimed in claim 1 in which the or each token is arranged to transmit continuously without requiring any action by the person carrying it.
 7. A security system as claimed in claim 1 additionally comprising a socket for each token and a battery charger for automatically charging batteries in the tokens when they are inserted in their sockets.
 8. A system as claimed in claim 1 in which the control unit includes a local oscillator oscillating at a frequency displaced from the predetermined frequency for producing beat frequency pulses and a detector circuit for distinguishing between beat frequencies above and below a certain limit.
 9. A system as claimed in claim 8 in which the beat frequency pulses are passed through a low pass filter in the control unit for cutting off beat frequencies outside a permitted range.
 10. A system as claimed in claim 9 including in the control unit a counter gate for counting beat frequency pulses over a predetermined time interval and means for detecting whether the count achieved is greater or less than a predetermined count.
 11. A system as claimed in claim 10 including means for the control unit for detecting whether no pulses are received over an interval and means connected to the counter gate and providing a wave signal for delimiting the inverval.
 12. A system as claimed in claim 10 including a logic circuit arranged to give an accept signal for controlling the deterrent in dependence on the count of the counter.
 13. A system as claimed in claim 12 wherein the logic circuit includes means for initiating a further detection cycle automatically in response to no accept signal being given within a predetermined time.
 14. A system as claimed in claim 13 including means for giving no alarm if, but only if, an acceept signal is given after at least two detection cycles.
 15. A system as claimed in claim 8 including in the control unit means for modulating the local oscillator frequency by a few cycles for a fraction of a detecting cycle and not for the remainder of the cycle, means providing a wave signal for delimiting the detecting cycle and means responsive to the wave signal for delimiting fractions of the cycle.
 16. A system as claimed in claim 15 including in the control unit gates for counting beat frequency pulses in one sense for the fraction of the cycle and in the other sense for the remainder of the cycle.
 17. A security system as claimed in claim 1 including a deterrent and means responsive to the control signal for operating the deterrent.
 18. A system as claimed in claim 17 in which the deterrent comprises a door having a lock arranged to be operated in response to the control signal from the control unit.
 19. A system as claimed in claim 17 in which the deterrent comprises an alarm arranged to be operated in response to the control signal from the control unit.
 20. A security system as claimed in claim 1 including an exit loop detector at an exit from premises and in which the control unit responds to a signal from the exit loop detector due to a token being taken through the exit.
 21. A security system for operating a deterrent comprising: a portable token for carrying within the system, the token having means for generating and transmitting oscillations at two different frequencies; a loop system at a certain location in the system for providing an output signal including the oscillation frequencies from the token, the loop system having two inductive loops in substantially perpendicular planes for picking up the oscillations from the token when the token is carried in any attitude near the location of the loop system and means combining the signals from the loops for providing the output signal; and a control unit receiving the output signal from the loop system for transmitting a signal operating the deterrent, the control unit having a detecting system, the detecting system having two detecting circuits each receiving the output signal from the loop system and each having a local oscillator arranged to oscillate at a frequency displaced from that of a different one of the token oscillation frequencies in the loop system output signal, means mixing the loop system output signal with the local oscillator frequency for forming beat frequency pulses, and a gate arranged to count the beat frequency pulses during at least part of a signal from a clock for passing a signal having a 0 or 1 state dependent upon the counting of a predetermined number of beat pulses during the clock signal, the control unit also having a clock for providing the clock signal to each gate and means responsive to the state of the signal from each gate for providing the signal operating the deterrent.
 22. A system as in claim 21 wherein the means combining the signals from the loops in the loop system comprise means for oppositely shifting the phase of the signal picked up by each loop and means adding each of the oppositely phase-shifted signals for providing the output signals. 